1. Field of the Invention
The present invention relates to a plasma treatment apparatus for treating objects with atmospheric pressure plasma, and a plasma generation method using the apparatus.
2. Disclosure of the Prior Art
In the past, various surface treatments are performed by use of atmospheric pressure plasma. For example, it is disclosed in each of Japanese Patent Early Publications [KOKAI] 2-15171, 3-241739 and 1-306569 that a plasma treatment is performed by a plasma treatment device comprising a pair of electrodes and a dielectric material disposed therebetween in a reaction chamber. In these methods, a plasma-generation gas containing a rare gas such as helium or argon as the main component is supplied into the chamber, and objects are treated with plasma generated by applying an AC electric field between the electrodes.
In addition, Japanese Patent Early Publications [KOKAI] 4-358076, 3-219082, 4-212253, 6-108257 and 11-260597 disclose about plasma treatment devices each using a plasma jet generated under a pressure in the vicinity of atmospheric pressure. For example, as shown in FIG. 8, a typical plasma treatment device of the prior art comprises a reaction tube 2P, gas supply unit 8P, a pair of first and second electrodes 3P, 4P disposed around the reaction tube, and an AC power source 11P connected to the first electrode 3P through an impedance-matching unit 12P. The second electrode 4P is grounded.
The reaction tube 2P is formed with a gas inlet 10P at its top opening and a plasma blowoff nozzle 1P at its bottom opening. The numeral 12P designates an impedance-matching unit 12P having a variable capacitor 14P and an inductor (not shown), which is used to obtain the impedance matching between the power source 11P and a plasma generation region 13P in the reaction tube 2P between the first and second electrodes 3P, 4P.
By using the above-described apparatus, a plasma treatment is performed as follows. First, the plasma-generation gas is supplied into the reaction tube 2P from the gas supply unit 8P through the gas inlet 10P, and then an AC electric field is applied between the first and second electrodes 3P, 4P to generate atmospheric pressure plasma in the reaction tube. The objects are treated by a jet-like plasma injected from the reaction tube 2P through the blowoff nozzle 1P.
In this plasma treatment, since the atmospheric pressure plasma is used, it is needed to apply a high voltage of more than 1 kV to the first electrode 3P for plasma generation. In addition, when the electrodes 3P, 4P are disposed outside of the reaction tube 2P, as shown in FIG. 8, most of the applied AC electric field is released into the surrounding space other than the interior of the reaction tube, so that a magnitude increase in discharge initiation voltage is required. Moreover, since a typical frequency of the AC electric field is 13.56 MHz, it is required to obtain the impedance matching between the power source 11P and the plasma generation region 13P.
Therefore, when such a high voltage is applied to the first electrode 3P to generate the atmospheric pressure plasma in the reaction tube 2P, arc discharge may occur in the variable capacitor 14P such as a conventional air capacitor of the impedance matching unit 12P. In this case, there is an inconvenience that the plasma can not be generated in the reaction tube and a malfunction of the plasma treatment apparatus is caused thereby. To solve this problem, it is proposed to use an expensive impedance matching unit with a vacuum capacitor having a high withstand voltage. However, there is another problem that the cost performance of the apparatus deteriorates.
In view of the above points, a primary object of the present invention is to provide a plasma treatment apparatus characterized in that plasma can be generated under a pressure in the vicinity of atmospheric pressure with reliability by help an ignition electrode to facilitate starting the apparatus without using an expensive impedance matching device. That is, the plasma treatment apparatus of the present invention comprises a plasma-generation chamber having an aperture from which the plasma blows out, a gas supply for supplying a gas for plasma generation into the chamber, a pair of electrodes, a power source for applying an AC electric field between the electrodes to maintain the plasma in the chamber, a pulse generator for providing a pulse voltage, and an ignition electrode for applying the pulse voltage to the gas supplied in the chamber to generate the plasma.
It is preferred that the pair of electrodes make contact with an outer surface of the chamber. In this case, since these electrodes are not exposed to the plasma, it is possible to prevent contamination of the plasma with the electrode substance.
It is also preferred that the ignition electrode is disposed in the vicinity of the aperture.
In a preferred embodiment of the present invention, the plasma treatment apparatus comprises an electrode traveling unit for traveling the ignition electrode between a first position where the ignition electrode is disposed in the vicinity of the aperture to apply the pulse voltage to the gas and a second position where the ignition electrode is spaced from the aperture. In this case, since the ignition electrode can be transferred from the first position to the second position after the plasma is generated, there is an advantage that the ignition electrode is out of the way of the plasma treatment.
It is preferred that the ignition electrode is disposed outside of the chamber and adjacent to a discharge region provided in the chamber by the pair of electrodes. In this case, since the ignition is not exposed to the plasma, it is possible to prevent contamination of the plasma with the electrode substance. It is also preferred that the ignition electrode makes contact with an outer surface of the chamber.
In a further preferred embodiment of the present invention, an inside dimension of the aperture is within a range of 1 mm to 20 mm. In this case, it is possible to improve the efficiency of the plasma treatment.
Another object of the present invention is to provide a plasma generation method using the plasma treatment apparatus described above comprising the step of applying a pulse voltage to a gas for plasma generation under a pressure in the vicinity of atmospheric pressure by use of the ignition electrode to generate plasma in the chamber.
In the plasma generation method, when a magnitude of the pulse voltage provided by the pulse generator is three times or more of a voltage value applied between the pair of electrodes, it is possible to stably generate the atmospheric pressure plasma.
These and still other objects and advantages will become apparent from the following detail description of the invention and examples of the invention.